WIP: ges: fix API export/import and 'inconsistent linkage' on MSVC

Export GES library API in headers when we're building the
library itself, otherwise import the API from the headers.

This fixes linker warnings on Windows when building with MSVC.

Fix up some missing config.h includes when building the lib which
is needed to get the export api define from config.h

Fixes gstreamer/gst-editing-services#42
6 jobs for win32-dll-linkage in 28 minutes and 39 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Preparation
passed gst indent #64095

00:00:33

passed manifest #64094

00:00:34

 
  Build
passed build fedora x86_64 #64096

00:03:24

 
  Test
passed check fedora #64097

00:15:09

 
  Full Builds
passed build android arm64 api28 #64098

00:03:29

passed build cerbero fedora x86_64 #64099

00:09:30